Easy-open can lid superior in can openability and process for production thereof

ABSTRACT

A can lid made of a metal sheet having a resin coating on at least one surface thereof and superior in can openability, having an outer peripheral flat portion and an inner peripheral flat portion straddling a score line further having an outer bead and inner bead at the sides of the outer peripheral flat portion and the inner peripheral flat portion opposite to the score line, and having a cross-section where the score line becomes gradually thinner from the thicknesses of the outer peripheral flat portion and the inner peripheral flat portion and a process for forming the same.

TECHNICAL FIELD

The present invention relates to a metal container can lid, moreparticularly to a metal easy-open can lid which enables part orsubstantially all of the can lid to be easily opened manually and to aprocess for producing the same. This can lid is used for beverage cansor general food cans and a wide range of other applications.

BACKGROUND ART

The easy-open can lids (or easy-open can lids) used for beverage cans,general food cans, etc. are made of surface-treated aluminum sheet orsteel sheet provided with score lines (or guide lines for opening) fortearing of the can lid to form an opening for removing the contents ofthe can.

At present, the method for forming a score line, as shown in FIG. 1, isto shape the can lid material 1 into the basic form of the can lid, thenplaced it on a flat die 2 and press down a die 3 having a projection ofthe shape of the contour of the opening to form a guide groove foropening (score) 4 of the shape of the opening in the material. Tofacilitate the can openability, it is necessary to press the die downuntil the score depth reaches about ½ to ⅔ of the thickness of the sheetbefore processing. However, if the depth of the score is too shallow,the can openability becomes poor, while if too deep, the strengthbecomes insufficient and the problem arises of the can opening intransport by a small impact from the outside. Therefore, there have beenthe problem that the projection of the die for forming the score isrequired to have a high precision in the shape of the angular tip of theprojection and the shape of the projection of the die changes anddeteriorates due to wear when forming the score.

On the other hand, as shown in FIG. 2, as in Japanese Unexamined PatentPublication (Kokai) No. 6-115548, Japanese Unexamined Patent Publication(Kokai) No. 8-224626, and Japanese Unexamined Patent Publication (Kokai)No. 9-108756, the process of production of an easy-open can lid bypressing the can lid material 1 by the shoulder portions of punchshoulder 8 and die shoulder 9 of upper and lower dies of an upper die(i.e., punch) 5 and lower die (i.e., die) 6 to form the score line hasbeen proposed. However, according to this method, since the contour ofthe score line is generally asymmetric, the deformation of the materialat the time of pressing did not become uniform and it was difficult toobtain a desired distribution of thickness across the entire length ofthe score line. That is, if trying to control the thickness of thethinnest portion of the score line (residual thickness) to be thin so asto improve the can openability, partially broken or extremely thinportions will be formed resulting in leakage of the contents andoccurrence of unnecessary opening due to impact at the time of transportetc. Further, if trying to control the residual thickness to be thick inorder to avoid breakage or occurrence of extremely thin portions, theproblem will remain that part of the score line will not tear even afteran opening operation, and therefore, the can openability will beinsufficient.

DISCLOSURE OF THE INVENTION

For the formation of the score of commercialized easy-open can lids, ascoring blade having an acute cross-section is used. Further, even inthe push-tab type, the tear strip is cut using a sharp cutting blade.There are problems in these in terms of productivity from the viewpointof the tool life. Further, particularly when using steel sheet as amaterial, the surface coating is broken by the processing and the metalis exposed, so repair coating is required. Further, when it comes torecycling, even among metal cans, while so-called “monometal cans” wherethe can body and the can lid are formed by the same material can be saidto be suitable for recycling, the present easy-open can lids are almostall made of aluminum. On the other hand, the bodies and can lids otherthan easy-open can lids are almost all made of steel sheet. Therefore,there has been a fervent desire for the development of some meansenabling production, with a good productivity, of easy-open can lidsmade of steel sheet which are superior in can openability and superiorin corrosion resistance.

An easy-open can lid in which a score line is formed by pressing theabove resin laminated metal sheet by the shoulder radii of upper andlower dies is meant to solve the above-mentioned problem, but practicalproblems remain in regard to the can openability and corrosionresistance.

As explained above, even with the method of pressing the shape of anopening by the shoulders of the upper and lower dies proposed forsolving the change and deterioration of the shape of the projection ofthe die due to wear at the time of scoring, there are still problemsremaining in the score line formed in the easy-open can lid due to theprocessing precision such as how to simultaneously achieve canopenability, air-tightness, and impact resistance. The present inventionachieves higher precision of control of the residual thickness at thetime of forming the score line by pressing by die shoulders.

In accordance with the present invention, there is provided a metaleasy-open can lid superior in can openability having a resin coating onat least one surface thereof, wherein said easy-open can lid has anouter peripheral flat portion and an inner peripheral flat portionstraddling a score line further having an outer bead and inner bead atthe sides of the outer peripheral flat portion and the inner peripheralflat portion opposite to the score line, and having a cross-sectionwhere the score line becomes gradually thinner from the thicknesses ofthe outer peripheral flat portion and the inner peripheral flat portion.

The easy-open can lid superior in can openability can have across-sectional shape where the gradually thinned thinly formed portionis bent.

In accordance with the present invention, there is further provided aneasy-open can lid superior in can openability wherein at least the innerbead is shaped projecting to the can outside, the deepest recess pointtoward the can inside of the inner peripheral flat portion is positionedtoward the can inside in a range of 0.2 to 4.0 mm at the can outsidesurface compared with the highest projecting point toward the canoutside of the inner bead, the interface portion of the outer peripheralflat portion and the score line is positioned toward the can outside ina range of 0.1 to 2.0 mm at the can outside surface compared with thedeepest recess point toward the can inside of the inner peripheral flatportion, and the widths of the inner peripheral flat portioncross-section and the outer peripheral flat portion cross-section arerespectively from 0.1 to 3.0 mm.

The easy-open can lid can be made of a metal sheet such as steel sheet,surface-treated steel sheet, aluminum sheet, or aluminum alloy sheet.The resin coating on the surface of the metal sheet at least at the caninner surface side can be made by a thermoplastic saturatedpolyester-based resin of a thickness of 10 to 100 μm. Further, the resincoating corresponding to the can outer surface can be made with apolyamide resin having a thickness of 10 to 100 μm.

The easy-open can lid according to the present invention, is either ofthe types where can lid does has not a tab for opening a tear stripprovided at least at one location of the can lid separating part or allof the can lid from the can body and the can lid does has not a tab foropening a tear strip provided at least at one location of the can lidleaving part or all of the can lid on the can body.

Further, the easy-open can lid may be opened by the method of opening atear strip provided at least at one location of the can lid, without atab, leaving part of the tear strip at the can body or separating itfrom the can body.

In accordance with the present invention, there is provided a method offorming an easy-open can lid superior in can openability comprising,when pressing the shape of the opening by shoulders of upper and lowerdies, pressing by using an upper die (punch) and lower die (die)provided with recesses at the die centers for restraining the shapingmaterial at the lower die and/or upper die, having at least one of thepunch shoulder R and die shoulder R of not more than 2.0 mm, and havingthe punch and die overlap on a parallel line of operation of the punch.

In the above method of formation, preferably the depth of the recess ofthe lower die (die) is not more than (punch shoulder R+die shoulder R)×4and the depth of the recess of the upper die (punch) head is at least0.2 mm.

In accordance with the present invention, there is further provided amethod of forming an easy-open can lid superior in can openabilitycomprising, when pressing the shape of the opening by shoulders of upperand lower dies, pressing by using upper and lower dies provided withrecesses at the die centers for restraining the shaping material at thelower die and/or upper die, having a shape of the shoulders of the dieand the punch of part of an ellipse having a long radius of 0.1 to 5.0mm and a short radius of 0.05 to 4.0 mm, and having the punch and dieoverlap on a parallel line of operation of the punch.

In the above method of formation, preferably the depth of the recess ofthe lower die (die) is not more than (punch shoulder R+die shoulder R)×4and the depth of the recess of the upper die (punch) head is at least0.2 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained in further detail below withreference to the drawings.

FIG. 1 is a view of the process of production of an can lid using a flatdie of the related art and a score having a V-shaped cross-section.

FIG. 2 is a view of the method of pressing the shape of an opening bythe shoulders of upper and lower dies of the related art.

FIG. 3 is a sectional view of the area near the score line of thepresent invention;

FIG. 4 is a view of an example of an easy-open can lid of the presentinvention.

FIG. 5 is a sectional view of the area near the score line of a firstaspect of the present invention;

FIGS. 6(A) and 6(B) are sectional views of the area near the score lineof a second aspect of the present invention.

FIGS. 7(A), 7(B), and 7(C) are sectional views of the area near thescore line of the second aspect of the present invention and the bentpart of the thinly formed portion.

FIG. 8 is a view of an example of an easy-open can lid of the secondaspect of the present invention.

FIG. 9 is a view of the process of production of a can lid of a thirdaspect of the present invention.

FIGS. 10(A), 10(B), 10(C), and 10(D) are views of typical easy-open canlids of the first aspect of the present invention.

FIGS. 11(A), 11(B), 11(C), 11(D), and 11(E) are views of the sectionalshapes of areas near the score line of the first aspect of the presentinvention.

FIGS. 12(A), 12(B), 12(C), and 12(D) are views of typical easy-open canlids of the second aspect of the present invention.

FIGS. 13(A), 13(B), 13(C), 13(D), and 13(E) are views of the sectionalshapes of the areas near the score line of the second aspect of thepresent invention.

FIG. 14 is a view of the outside surface of a typical easy-open can lidproduced by the third aspect of the present invention.

FIG. 15 is a view of the effects of the depth of the recess of the punchhead on the variations in the residual thickness in the embodiments.

FIG. 16 is a view of the effects of the depth of the recess of the diecenter on the variations in the residual thickness in the embodiments.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in further detail.

First, a first aspect of the present invention will be explained. Thepresent invention, as shown in FIG. 3, provides an easy-open can lidmade of a metal sheet free from repair of the inner and outer surfacesand provided with both corrosion resistance and can openability bymaking the sectional shape of the area near the score line 12 one havingan outer peripheral flat portion and inner peripheral flat portionbetween an outer bead 13 and inner bead 16 and having a score line 12 ofa shape which becomes gradually thinner between the outer peripheralflat portion 14 and the inner peripheral flat portion 15.

The sectional shape is not necessarily required along the entire lengthof the score line 12. It is sufficient if at least 30% of the score line12 has that shape. Further, depending on the relationship with theposition of the opening, the countersink or clinched portion may be usedinstead of the outer bead.

The flat portions 14 and 15 spoken of here mean flat portions includingportions of a radius of curvature of at least 50 mm. Further, the lengthof the flat portions is called the length of the width of the portionhaving a radius of curvature of at least 50 mm.

In forming the score line 12, since the resin coating is broken bypressing by a sharp blade according to the related art, repair coatingis required, and therefore, the present invention has a graduallythinned score line 12. Becoming gradually thinner means the sheetthickness has a rate of change of not more than 1000 μm/mm at smallportions where the thickness changes. More preferably a range of 250 to750 μm/mm is preferable.

This is because when forming a score line 12 of a rate of change of morethan 1000 μm/mm, defects occur in the resin coating and poorly bondedportions of the coating occur.

To ensure a stable, sound coating, not more than 750 μm/mm ispreferable. Further, if less than 250 μm/mm, the processed portionreduced in thickness becomes too long, and therefore, the portionreduced in bonding with the coating becomes broader. If the reduction inthickness becomes too gradual, stress tends to concentrate at the timeof opening and there are concerns about the can openability. Therefore,a range of 250 to 750 μm/mm is preferable. Such a gradually thinnedscore line 12 can be formed by pressing by the shoulders of upper andlower dies.

As proposed in Japanese Unexamined Patent Publication (Kokai) No.6-122438, in the case of an easy-open can lid obtained by thinning ametal sheet by pressing by the shoulder radii of upper and lower diesand then further pushing back the sheet to form a V-shaped bent portionto make the score, the can openability is improved. When the thinnedportion is formed into a V-shape by pushing back, the bonding of theresin coating at that portion easily falls, and therefore, the problemremains that careful care is required to prevent corrosion or rusting.

Therefore, the present invention only processes the area near thegradually thinned score line 12 to make it gradually thinner and formsthe score line 12 between the outer peripheral flat portion 14 remainingas the can lid and the inner peripheral flat portion 15 forming theopening portion so as to ensure the bonding of the resin coating in thearea of the score line 12. Further, beads 13 and 16 are provided at theoutside of the outer peripheral flat portion 14 and the inside of theinner peripheral flat portion 15. Due to this, stress easilyconcentrates at the score line 12 when giving stress for opening thecan. In pressing by a sharp blade of the related art, a V-shaped grooveis formed and the can is opened by a shear force. In the case of thegradually thinned score line 12, the can is opened by tensile breakageof the metal sheet. This is because by giving the two beads 13 and 16,it is possible to effectively utilize the stress for opening the can fortensile breakage of the sheet along the score line 12.

In particular, in the case of a stay-on type easy-open can lid, part ofthe opening is broken (initially broken) by pulling the tab and then thetab is further pulled to fully open the can. However, the initialbreakage is difficult. If the stroke of the tab is used up for theinitial breakage, there is no longer any tab stroke for opening the canand there is the problem that the can is only half opened.

As shown in the present invention, when initial breakage occurs due topulling, if the area near the score line 12 is V-shaped or S-shaped, thetab stroke is sometimes used up for the deformation of the V-shape orS-shape.

The present invention makes effective use of the tab stroke bypositioning the score line 12 between the inside and outer beads andgradually thinning the area near the score line 12.

In particular, in the case of the easy-open can lid 10 of the stay-ontype shown in FIG. 4, the inner bead and outer bead are shapedprojecting to the can outer surface side, the inner peripheral flatportion has a width of the recessed flat portion (wi=) at the can innersurface side of 0.1 to 3.0 mm in a range of hi=0.2 to 4.0 mm comparedwith the inner bead and the outer peripheral flat portion has a width ofthe flat portion (wo=) of 0.1 to 3.0 mm in the range of 0.1 to 2.0 mmfrom the inner peripheral flat portion (ho=).

If wi and wo are less than 0.1 mm, the bonding of the coating at thescore line portion falls and a problem results in terms of the corrosionresistance. Further, if more than 3.0 mm, the distance between the beadand the score line becomes too great and a problem arises in the canopenability. Due to this, wi and wo preferably are in a range of 0.1 to3.0 mm.

If hi is less than 0.2, the rigidity of the tear strip becomes smallerand there is a tendency for the pushdown force of the tab not to beeffectively utilized. If ho is less than 0.1 mm, the vector of the forcerequired for causing the initial breakage undesirably becomesoff-centered.

If ho is more than 2.0 mm, when transporting the easy-open can lidsstacked on each other, flaws are caused due to the upper and lower canlids rubbing against each other. Further, even if hi is more than 4.0mm, there is the same type of problem in transport.

Using the panel portion of the lid body as the horizontal standard, theheight and length are calculated by the method shown in FIG. 5. Theouter bead may be a projection facing the can inside direction. Notethat the height or depth of the outer bead is preferably in the range of(h_(B)=) 0.2 to 3.0 mm. Of course, the above shape an be used for othertypes of easy-open can lids such as pull-tab types, full-open types, andpushdown types.

If h_(B) is less than 0.2 mm, the rigidity of the can lid body sidebecomes smaller and there is an undesirable tendency for the pushdownforce of the tab at the time of can opening not being effectively used.Further, if h_(B) is more than 3.0 mm, when transporting the easy-opencan lids stacked on each other, flaws are caused due to the upper andlower can lids rubbing against each other.

The steel sheet is usually one having a thickness t₀ of 0.080 to 0.280mm and having mechanical properties of a hardness (H_(R30T)) of 46 to 68and an elongation of 10 to 60% or so.

The surface of the steel sheet is preferably plated by one or more ofSn, Cr, Ni, Al, and Zn. Chromium-treated steel sheet is preferable.

As the sheet steel specifically used, there are a tin-plated sheet steelgiving tin-plating of 0.5 to 3.0 g/m² and then chemical treatment, anickel-plated steel sheet giving a nickel plating of 0.3 to 2.0 g/m² andthen chemical treatment, an Sn/Ni-plated sheet steel giving a Ni andthen Sn plating of 0.5 to 2.0 g/m² and 0.01 to 0.5 g/m², respectively,then chemical treatment, and a chrome-chromate treated sheet steelnormally called TFS (Tin Free Steel) giving a metal Cr deposition of 50to 200 mg/m² and a chrome oxide deposition of 5 to 30 mg/m², in terms ofchrome.

Further, the aluminum sheet used in the present invention ordinarily hasa thickness t₀ of 0.18 to 0.32 mm. As the alloy, 5052, 5082, 5182, 5352,5349, and 5017 and a temper of H19 are preferable.

A surface-treated metal sheet comprising this aluminum sheet treated bychromate, treated by zirconate, or chemically treated by aphosphate-chromate system may also be used. A resin coating is necessaryon both surfaces of the metal sheet to ensure the corrosion resistanceand rust resistance. The laminated resin at least at the can innersurface side is preferably a saturated polyester-based resin coating ofa thickness of 10 to 100 μm from the viewpoints of economy, corrosionresistance, and flavor.

The saturated polyester-based resin in the present invention means alinear thermoplastic polyester obtained by condensation polymerizationof a dicarboxylic acid and diol and is best represented by polyethyleneterephthalate. As the dicarboxylic acid component, there areterephthalic acid, isophthalic acid, phthalic acid, adipic acid, sebacicacid, azelaic acid, 2,6-naphthalene dicarboxylic acid, decanedicarboxylic acid, dodecane dicarboxylic acid, cyclohexane dicarboxylicacid, and the like alone or in mixtures. As the diol component, thereare ethylene glycol, butane diol, decane diol, hexane diol, cyclohexanediol, neopentyl glycol, and the like alone or in mixtures. Copolymers oftwo or more dicarboxylic acid components or diol components orcopolymers with diethylene glycol, triethylene glycol, and othermonomers or polymers are also possible.

Further, when clinching the easy-open can lid on the can body, the resincoating is shaved off and problems arise in workability and appearancein some cases. From the viewpoint of the clinchability, the resincoating of the outer surface is preferably a polyamide resin. As thepolyamide resin, there are nylon 6, nylon 12, nylon 5, nylon 11, and thelike alone or in mixtures.

Further, the metal sheet resin coating used in the present invention mayin accordance with need have blended into it a plasticizer, antioxidant,thermal stabilizer, inorganic particles, pigments, organic lubricants,and other additives.

The thickness of the resin coating at the unprocessed part of the metalsheet used in the present invention is preferably 10 to 100 μm.

The present invention has a score line comprised of a gradually thinnedpart of the metal sheet by processing such as pressing by upper andlower dies. Since the resin coating is thinned together with the metalsheet, if the thickness of the resin is less than 10 μm, the barrierproperty (corrosion resistance and rust resistance) of the resin coatingat the processed part cannot be secured. If more than 100 μm, the effecton the barrier property of the resin coating becomes saturated andeconomic disadvantages are incurred. When considering the stability ofthe performance, economy, etc., a thickness of a range of 16 to 60 μm isparticularly effective.

The thickness of the metal sheet at the portion of the score line ispreferably 10 to 75 μm in the case of steel sheet and 35 to 130 μm inthe case of aluminum sheet in view of securing can openability andsecuring dropping strength, that is, if the thickness of the metal sheetis thick, the can openability falls, while if it is thin, the droppingstrength falls. More preferably, a range of 20 to 60 μm is preferable inthe case of steel sheet and a range of 35 to 125 μm in the case ofaluminum sheet.

In this series of processing steps, the resin coating having the abovecharacteristics is drawn uniformly together with the base material.Further, according to the process of the present invention, theprocessing is based on extrusion or pushback by the smoothly curvedshoulder portions of the projections, so there is almost none of theproblem of tool life seen in the method of pressing by a sharp blade. Asuperior productivity is guaranteed and an easy-open can lid isobtained.

A second aspect of the present invention will be explained in detailbelow.

First, the shape will be explained. The present invention, as shown inFIG. 6(A) and FIG. 6(B), provides an easy-open can lid made of a metalsheet and free from repair of the inner and outer surfaces given bothcan openability and corrosion resistance by making the cross-sectionalshape near the score line (bent portion) 12 one having an outerperipheral flat portion 14 and an inner peripheral flat portion 15between the outer bead 13 and inner bead 16, further having a score line12 of a shape which becomes gradually thinner between the outerperipheral flat portion 14 and the inner peripheral flat portion 15, andbent. This cross-sectional shape does not necessarily have to extendover the entire length of the score line 12. It is sufficient if atleast 30% of the score line 12 has this shape. Further, depending therelationship with the position of the opening, the countersink orclinched portion may be used instead of the outer bead 13.

The flat portions 14 and 15 spoken of here mean flat portions includingportions of a radius of curvature of at least 50 mm. Further, the lengthof the flat portions is called the length of the width of the portionhaving a radius of curvature of at least 50 mm.

In forming the score line (bent portion) 12, since the resin coating isbroken by pressing by a sharp blade according to the related art, repaircoating is required, so the present invention has a gradually thinnedscore line 12. Becoming gradually thinner means the sheet thickness hasa rate of change of not more than 1000 μm/mm at small portions where thethickness changes. More preferably a range of 250 to 750 μm/mm ispreferable.

This is because when forming a score line 12 of a rate of change of over1000 μm/mm, defects occur in the resin coating and poorly bondedportions of the coating occur.

To ensure a stable, sound coating, not more than 750 μm/mm ispreferable. Further, if less than 250 μm/mm, the processed portionreduced in thickness becomes too long, so the portion reduced in bondingwith the coating becomes broader. If the reduction in thickness becomestoo gradual, stress tends to concentrate at the time of can opening andthere are concerns about the can openability. Therefore, a range of 250to 750 μm/mm is preferable. Such a gradually thinned score line can beformed by pressing by the shoulders of upper and lower dies.

In the case of an easy-open can lid having a score of a V-shaped grooveformed by pressing by a sharp blade of the related art, the can isopened by a shear force. However, in the case of the gradually thinnedscore line as in the present invention, the can is opened by tensilebreakage of the metal sheet. Giving the two beads enables the stress dueto the can opening to be effectively utilized for the tensile breakageof the score line and improves the can openability. Further, due to thecross-sectional shape where the portion formed thinly in thickness isbent, the stress for opening the can easily concentrates due to the bentshape and enables the can openability to be further improved. The angleof the bend is more preferably from 20 degree to 90 degree. If more than90 degree, the bonding of the resin coating falls in the thinningprocess. The bending further causes the bonding of the resin coating tofall and makes it necessary to take careful care so that corrosion orrust does not occur. Further, if less than 20 degree, the extent ofimprovement of the can openability becomes smaller.

Note that the angle 19 of the bent portion may be found as the angle ofthe intersection with the center line 18 of the base material at the twosides of the bent portion as shown in FIG. 7(B). Next, an explanationwill be made of an example of the method of drawing the center line byFIG. 7(C). First, the peak of the curved portion 12 is determined. Theportion of the smallest radius of curvature of the surface of the metalsheet at the can inner surface side of the bent portion 12 is made Gi,the portion of the smallest radius of curvature of the surface of themetal sheet at the can outer surface side is made Go, and the centerpoint of the line segment Gi-Go is made the peak C. An arc of a distanceof the thickness t₀ of the metal sheet before processing from the peak Cis drawn, the intersection with the surface of the metal sheet at thecan inner surface side is made Xi, and the intersection with the surfaceof the metal surface of the can outer surface side is made Xo. The linepassing through the peak C and the center point M of the line segmentXi-Xo is made the center line 18. Note that as the center line betweenpeaks in the case where there are a plurality of peaks, the lineconnecting the peaks is made the center line. Further, as shown in FIG.6(B) and FIG. 7(B), there is no problem even if a plurality of bentportions are formed at the portion of the sheet formed thinner.

In particular, in the case of an easy-open can lid 10 of the stay-ontype shown in FIG. 8, as shown in FIG. 7(A), the inner bead 16 and theouter bead 13 are shaped projecting to the can outer surface side. Theinner peripheral flat portion 15 has a width of the recessed flatportion (wi) at the can inner surface side of 0.1 to 3.0 mm in the rangeof hi=0.2 to 4.0 mm compared with the inner bead 16, while the outerperipheral flat portion 14 has a width of the flat portion (wo) of 0.1to 3.0 mm in the range of 0.1 to 2.0 mm from the inner peripheral flatportion 15 (ho). If wi and wo are less than 0.1 mm, the bonding of thecoating at the score line 12 portion falls and a problem results interms of the corrosion resistance. Further, if more than 3.0 mm, thedistance between the bead and the score line 12 becomes too great and aproblem arises in the can openability. Due to this, wi and wo preferablyare in a range of 0.1 to 3.0 mm.

If hi is less than 0.2 mm, the rigidity of the tear strip becomessmaller and there is a tendency for the pushdown force of the tab not tobe effectively utilized. If ho is less than 0.1 mm, the vector of theforce required for causing the initial breakage undesirably becomesoff-centered.

If ho is more than 2.0 mm, when transporting the easy-open can lidsstacked on each other, flaws are caused due to the upper and lower canlids rubbing against each other. Further, even if hi is more than 4.0mm, there is the same type of problem in transport.

Using the panel portion of the can body as the horizontal standard, theheight and length are calculated by the method shown in FIG. 7(A). Theouter bead 13 may be a projection facing the can inside direction. Notethat the height or depth of the outer bead 13 is preferably in the rangeof (h_(B)=) 0.2 to 3.0 mm. Of course, the above shape can be used forother types of easy-open can lids such as pull-tab types, full-opentypes, and pushdown types.

If h_(B) is less than 0.2 mm, the rigidity of the can lid body sidebecomes smaller and there is an undesirable tendency for the pushdownforce of the tab at the time of can opening not being effectively used.Further, if h_(B) is more than 3.0 mm, when transporting the easy-opencan lids stacked on each other, flaws are caused due to the upper andlower can lids rubbing against each other.

The steel plate, aluminum sheet, polyester resin, polyamide resin, resincoating, and method of processing used in the present invention are thesame as the first aspect of the present invention explained above.

A third aspect of the present invention will be explained in detailbelow. First, the basic technical idea of the present invention will beexplained.

The present inventors found that the inability to uniformly absorb theexcess material caused at the material at the inside of the score linewhen pressed by die shoulders is one cause for the nonuniformity of theresidual thickness. One reason for this is believed to be that thematerial at the inside of the score line at the time of pressing is notrestrained by the die and free deformation is allowed. In the presentinvention, the left over material at the inside of the score line, asshown in FIG. 9, limits the space for the escape of the material by thespace (recess) 20 formed in the die (punch) 5 at the inside of the scoreline or the space (recess) 20 formed in the dies (die) 6, 7 at the sameside of the score line. Due to this, preferable control of the residualthickness over the entire length of the score line becomes possible. Theimportant requirement of the present invention, as shown in FIG. 9, isthat the width a of the flat portion from the punch shoulder to thepunch head recess, the overlapping width b of the punch and die on theline parallel to the operation, the punch head recess c, and the diecenter recess depth d be suitably defined, whereby the shape of theescape portion of the material and the shape of the restraining portionof the material at the inside of the score line are suitably limited.Further, the radii of the die shoulders, clearance, residual thickness,etc. are the same as in the related art.

Below, a detailed explanation will be given of the reasons for thenumerical limitations in the present invention.

When forming the shape of the opening portion by pressing by a diehaving part of an arc at the shoulder shape of the upper and lower dies,it is necessary that at least one of the punch shoulder R and the dieshoulder R be not more than 2.0 mm. This is because when the shoulder Rare too large, the score line becomes thicker and the concentration ofstress on the score line disperses and the can openability is reduced.The lower limit is not particularly set, but 0.05 mm or more ispreferable to maintain the soundness of the film.

Further, when forming the shape of the opening portion by pressing byupper and lower dies having part of an ellipse for the shape of theshoulders of the dies, it is necessary to make the long radius of theellipse forming part of the shapes of the shoulders of the punch and diefrom 0.1 mm to 5.0 mm and the short radius from 0.05 mm to 4.0 mm.

If the long radius of the ellipse forming part of the shapes of theshoulders of the punch and die is more than 5.0 mm or the short radiusis more than 4.0 mm, the score line becomes thick, the concentration ofstress on the score line is dispersed, and the can openability isreduced.

Further, if the long radius of the ellipse forming part of the shape ofthe shoulders of the punch and die is less than 0.1 mm or the shortradius is less than 0.05 mm, when using a metal sheet having a resincoating laminated on the can lid base material, it is not possible tomaintain the soundness of the resin coating.

The punch head is formed with a recess for allowing the left overmaterial to escape at the inside of the score line by pressing, but thedepth is made 0.2 mm or more. Further, the depth of the recess at thecenter of the die is made not more than the (long radius of punchshoulder+long radius of die shoulder)×4. These values are important forcontrolling the escape of the material. In particular, the depth of therecess of the center of the die has to be controlled so that the bottomof the recess contacts the material at the time of processing. The limitof the depth depends on the amount of pressing, but it can be estimatedas follows: That is, due to the reduction of the thickness at the timeof pressing by the punch and die shoulders, the length of the score linebecomes longer by about the extent of the (long radius of the punchshoulder+long radius of the die shoulder) in the vertical direction.

The outside of the score line is restrained by the beads etc., andtherefore, it is believed that almost no movement of the materialoccurs. Therefore, material of about the (long radius of the punchshoulder+long radius of the die shoulder) occurs as excess material atthe inside of the score line. The bulging of the material due to thisprobably becomes several times the (long radius of the punchshoulder+long radius of the die shoulder).

Regarding the determination of the various conditions, the effect of theratio between the depth of the recess of the center of the die and the(long radius of the punch shoulder+long radius of the die shoulder) onthe uniformity of the residual thickness was studied. As a result, itwas found that if the depth of the recess of the center of the diebecomes more than four times the (long radius of the punch shoulder+longradius of the die shoulder), the variation of the residual thicknessbecomes larger. That is, if over this extent of depth, the effect of therestraint of the material by the recess at the center of the die doesnot appear, that is, the bottom of the recess and the material no longcontact each other. If there is no such contact, the escape of thematerial will not be controlled, the nonuniformity of the residualthickness will be greater like in the related art, and it will not bepossible to satisfy the opening performance. Further, in the final stageof the pressing, the material guided into the punch recess contacts thebottom of the recess. When suitably pushed back, a pushback is given tothe then thinnest score line and stress easily concentrates there at thetime of opening operation, and therefore, the can openability isimproved.

The width of the overlap of the punch and die on the line parallel tothe operation of the punch is made less than the long radius of thepunch shoulder+the long radius of the die shoulder. This is to form thesectional shape of the score line with a good can openability. If thereis no overlapping width, shear deformation results, and therefore, itbecomes difficult to form an extremely thin portion without breakage.Further, if the width becomes too great, the score line becomes thick,the concentration of stress on the score line is dispersed, and the canopenability is reduced. While depending on the ductility of thematerial, it is preferable to make the overlapping width at least (longradius of punch shoulder+long radius of die shoulder)/2 to avoid therisk of breakage of the material due to shear deformation.

Further, in actual production, the width of the flat portion from thepunch shoulder to the punch head recess changes over the entire lengthof the score line, but the maximum width is preferably made not morethan 6.0 mm. This is to facilitate the escape of the excess material tothe punch head recess. To restrain the escape of the excess material tothe punch head recess to a certain extent, it is preferable to make themaximum width of the flat portion not less than 0.5 mm. Note that thematerial of the opening can lid using the present invention is notparticularly limited. The effect can be obtained by an aluminum alloysheet, steel sheet, or surface-treated sheets of the same or such sheetslaminated with a resin film.

EXAMPLES

Examples of the present invention will be explained below together withthe Comparative Examples.

Example I and Comparative Example I

The types (see FIG. 10), the sectional shape (see FIG. 11), and thematerials of the easy-open can lids, the thicknesses, materials, resincoatings of the score line, and the results of the performanceevaluation are shown in Table I-1.

FIG. 10(A) shows a stay-on tab type easy-open can lid, FIG. 10(B) showsa pull-tab type easy-open can lid, FIG. 10(C) shows a tabless typeeasy-open can lid, and FIG. 10(D) shows a full-open type easy-open canlid.

FIGS. 11(A) and 11(B) are sectional views of the areas near the scorelines of the present invention, while FIGS. 11(C), 11(D), and 11(E) showsectional views of the areas near the score line of the ComparativeExample.

For the evaluation, the corrosion resistance, can openability, anddropping strength were investigated.

The corrosion resistance was evaluated, as an evaluation of the caninner surface side, by filling the can with an aqueous solution ofhydrochloric acid+iron chloride (75 cc of hydrochloric acid+150 g ofFeCl₂+1925 cc of water), storing the can at 50° C. for one month, andexamining it visually.

For the can outer surface side, the can was immersed in tap water atroom temperature for one month and the state of occurrence of rust wasvisually examined.

Note that the can openability was judged by whether the can could befully opened without problem and by measuring the opening strength. Thedropping strength was evaluated by fabricating a can with an easy-opencan lid filled with a liquid, dropping it vertically from a height of 60cm five times, and examining the state of leakage of the contents fromthe easy-open can lid due to the dropping impact by n=50 times.

The evaluations of “extremely”, “good”, “practical”, and “largevariation” were made as in Table I-1.

With tab: FIG. 10(A): stay-on tab type easy-open can lid, FIG. 10(B):pull-tab type easy-open can lid, and FIG. 10(D): full-open typeeasy-open can lid.

Without tab: FIG. 10(C): tabless type easy-open can lid

TABLE I-1 Ex. I-1 Ex. I-2 Ex. I-3 Ex. I-4 Ex. I-5 Ex. I-6 Ex. I-7 Ex.I-8 Ex. I-9 Can lid shape Type of easy-open can lid FIG. 10C FIG. 10AFIG. 10A FIG. 10A FIG. 10B FIG. 10D FIG. 10C FIG. 10A FIG. 10A Sectionalshape FIG. 11A FIG. 11A FIG. 11B FIG. 11A FIG. 11A FIG. 11A FIG. 11AFIG. 11A FIG. 11A hi (mm) 2.0 1.0 1.0 0.5 3.5 2.5 0.6 0.2 1.0 wi (mm)2.0 0.3 1.5 0.2 2.0 3.5 0.2 2.0 1.5 ho (mm) 1.0 0.2 0.5 0.1 1.4 0.9 0.40.2 0.5 wo (mm) 2.0 0.3 0.5 0.5 2.5 1.5 0.3 2.0 0.5 Thickness of metalsheet at  30  45  65 100 120 150  7  15  25 score line (μm) Rate ofchange of sheet 500 550 500 450 150 300 500 550 500 thickness of scoreline portion (μm/mm) Metal sheet Al sheet Al sheet Al sheet Al sheet Alsheet Al sheet Steel Steel Steel sheet sheet sheet Sheet thickness (nm)0.23 0.25 0.25 0.25 0.25 0.23 0.19 0.19 0.19 Type 5082 5017 5052 51825352 5349 TFS CL TFS series series series series series series Weight(mg/m²) Zirconium film¹⁾ 10 10 — — — — — — — Hydrated Cr oxide²⁾ — — 5040 45 —  17  25  17 Metal Cr — — — — — — 100  15 100 Sn — — — — — — —1050 — Ni — — — — — — —  20 — Resin film Inside surface Resin filmPoly-ester Poly-ester Poly-ester Poly-ester Poly-ester Poly-esterPoly-ester Poly-ester Poly-ester Film thickness (μm)  9 40 30 20 30 1260 60 80 Outside surface Resin film Nylon 6 Nylon 12 Nylon 66 Poly-esterNylon 6 Nylon 6 Nylon 6 Nylon 6 Nylon 6 Film thickness (μm)  9 40 30 2030 12 60 60 80 Performance evaluation Corrosion resistance P EG EG EG GC P G EG Can openability EG EG EG EG G P EG EG EG Initial opening force(kg) or 3.5 1.5 1.8 2.1 2.3 3.0 3.6 1.6 full can openability Droppingstrength P G EG EG EG EG P G EG No. of leaking cans/50 cans 8 4 0 0 0 09 3 0 Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. Ex. I-10 Ex. I-11 Ex. I-12Ex. I-13 Ex. I-14 I-1 I-2 I-3 I-4 Can lid shape Type of easy-open canlid FIG. 10A FIG. 10B FIG. 10D FIG. 10A FIG. 10A FIG. 10A FIG. 10A FIG.10A FIG. 10A SOT SOT SOT SOT Sectional shape FIG. 11A FIG. 11A FIG. 11AFIG. 11A FIG. 11A FIG. 11C FIG. 11D FIG. 11E FIG. 11A hi (mm) 0.5 3.50.6 5.0 2.0 — — — 2.0 wi (mm) 0.2 2.0 0.2 0.05 1.5 — — — 2.0 ho (mm) 0.11.4 0.4 2.5 −0.3  — — — 1.0 wo (mm) 0.5 2.5 0.3 0.5 1.5 — — — 2.0Thickness of metal sheet at  45  55  80  40  45  40  40  40  45 scoreline (μm) Rate of change of sheet 450 950 700 500 450 500 500 500 1200thickness of score line portion (μm/mm) Metal sheet Steel Steel SteelSteel Steel Steel Steel Steel Steel sheet sheet sheet sheet sheet sheetsheet sheet sheet Sheet thickness (mm) 0.19 0.19 0.19 0.19 0.19 0.190.19 0.19 0.19 Type TFS TFS ET TFS TFS TFS TFS TFS TFS Weight (mg/m²)Zirconium film¹⁾ — — — — — — — — — Hydrated Cr oxide²⁾  17  17  12  17 17  17  17  17  17 Metal Cr 100 100  12 100 100 100 100 100 100 Sn — —2650 — — — — — — Ni — — — — — — — — — Resin film Inside surface Resinfilm Poly-ester Poly-ester Poly-ester Poly-ester Poly-ester Poly-esterPoly-ester Poly-ester Poly-ester Film thickness (μm) 30 12 9 30 30 30 3030 30 Outside surface Resin film Nylon 6 Nylon 6 Nylon 6 Nylon 6 Nylon 6Nylon 6 Nylon 6 Nylon 6 Nylon 6 Film thickness (μm) 30 12 9 30 30 30 3030 30 Performance evaluation Corrosion resistance EG P P P P EG LV LV LVCan openability EG G P EG G LV EG EG EG Initial opening force (kg) or2.1 2.5 3.0 2.0 2.5 Can't open 2.1 2.1 2.2 full can openability Droppingstrengtn EG EG EG EG EG EG EG EG EG No. of leaking cans/50 cans 0 0 0 00 0 0 0 0 TFS: Chrome-chromate treated steel sheet, ET: Electrical tinplated steel sheet, CL: Ni substrate thin Sn plated steel sheet ¹⁾Shownas value converted to zirconium. ²⁾Amount of hydrated Cr oxide showsamount as Cr. Evaluation of corrosion resistance: EG: Extremely good, C:Good, P: Practical, LV: Large variation (unstable)

TABLE I-2 Extremely Large good Good Practical variation Can Under 2.22.3 to 2.8 to No full openability kg, full 2.7 kg, 3.2 kg, opening (withtab) opening full full at over Initial force opening opening 3.3 kgopening force Possibility of full opening (without tab) Under 3.7 3.8 to4.3 to Over 4.8 Initial kg 4.2 kg 4.7 kg kg opening force Dropping None1 to 4 5 to 9 10 or strength more (number of leaking cans)

Example II and Comparative Example II

The types (see FIG. 12), the sectional shape (see FIG. 13), and thematerials of the easy-open can lids, the thicknesses of the thinnestportions of the metal sheet, materials, resin coatings of the scoreline, and the results of the performance evaluation are shown in TableII-1.

FIG. 12(A) shows a stay-on tab type easy-open can lid, FIG. 12(B) showsa pull-tab type easy-open can lid, FIG. 12(C) shows a tabless typeeasy-open can lid, and FIG. 12(D) shows a full-open type easy-open canlid.

FIGS. 13(A) and 13(B) are sectional views of the areas near the scorelines of the present invention, while FIGS. 13(C), 13(D), and 13(E) showsectional views of the areas near the score line of the ComparativeExample.

For the evaluation, the corrosion resistance, can openability, anddropping strength were investigated.

The corrosion resistance was evaluated, as an evaluation of the caninner surface side, by filling the can with an aqueous solution ofhydrochloric acid+iron chloride (75 cc of hydrochloric acid+150 g ofFeCl₂+1925 cc of water), storing the can at 50° C. for one month, andexamining it visually.

For the can outer surface side, the can was immersed in tap water atroom temperature for one month and the state of occurrence of rust wasvisually examined.

TABLE II-1 Ex. II-1 Ex. II-2 Ex. II-3 Ex. II-4 Ex. II-5 Ex. II-6 Ex.II-7 Ex. II-8 Ex. II-9 Can lid shape Type of easy-open can lid FIG. 12AFIG. 12C FIG. 12A FIG. 12A FIG. 12B FIG. 12D FIG. 12A FIG. 12C FIG. 12CSectional ahape FIG. 13A FIG. 13A FIG. 13B FIG. 13A FIG. 13A FIG. 13AFIG. 13A FIG. 13A FIG. 13B hi (mm) 2.0 3.7 0.3 0.5 3.5 0.1 2.0 3.7 4.5wi (mm) 1.5 0.2 2.9 0.2 2.0 3.2 1.5 0.2 0.05 ho (mm) 1.0 1.8 0.05 0.21.4 2.2 1.0 1.9 2.5 wo (mm) 1.5 0.2 0.5 2.5 3.2 0.2 1.5 0.3 0.05Thickness of metal sheet at  90  45 100 100 120 120  25  8  15 scoreline (μm) Rate of change of sheet 500 725 500 200 950 300 500 260 300thickness of score line portion (μm/mm) Angle (°) of bending of  45  25 15 175  50  45  85 105  25 thinned portion Metal sheet Al sheet Alsheet Al sheet Al sheet Al sheet Al sheet Steel Steel Steel sheet sheetsheet Sheet thickness (mm) 0.23 0.25 0.25 0.25 0.25 0.23 0.19 0.19 0.19Type 5052 5017 5052 5182 5352 5349 series aeriea series series seriesseries TFS TFS TFS Weight (mg/m²) Zirconium film¹⁾ 10 10 — — — — — — —Hydrated Cr oxide²⁾ — — 50 40 45 —  17  16  17 Metal Cr — — — — — — 100115 100 Sn — — — — — — — — — Ni — — — — — — — — — Resin film Insidesurface Resin film Poly-ester Poly-ester Poly-ester Poly-esterPoly-ester Poly-ester Poly-ester Poly-eater Poly-ester Film thickness(μm) 25 55 30 20 30 12 40 95 60 Outside surface Resin film Nylon 6 Nylon12 Nylon 66 Poly-ester Nylon 6 Nylon 6 Nylon 6 Nylon 6 Nylon 6 Filmthickness (μm) 25 55 30 20 30 12 40 95 60 Performance evaluationCorrosion resistance EG EG EG P G G EG G EG Can openability EG EG G EG GG EG EG G Initial opening force (kg) or 1.5 3.5 2.3 2.1 2.3 2.7 1.8 3.64.0 full can openability Dropping strength EG EG EG EG EG EG EG P G No.of leaking cans/50 cans 0 0 0 0 0 0 0 5 1 Ex.II-10 Ex. II-11 Ex.II-12Ex. II-13 Ex. II-14 Comp. Ex. Comp. Ex. Comp. Ex. Comp. Ex. II-1 II-2II-3 II-4 Can lid shape Type of easy-open can lid FIG. 12A FIG. 12B FIG.12D FIG. 12A FIG. 12A FIG. 12A FIG. 12A FIG. 12A FIG. 12A SOT SOT SOTSOT Sectional shape FIG. 13A FIG. 13A FIG. 13A FIG. 13A FIG. 13A FIG.13C FIG. 13D FIG. 13E FIG. 13A hi Cmm) 0.5 0.2 0.6 5.0 2.0 — — — 2.0 wi(mm) 0.2 3.1 0.2 0.05 1.5 — — — 2.0 ho (mm) 0.2 1.4 0.4 2.5 −0.3  — — —1.o wo (mm) 0.5 3.3 0.3 0.2 2.5 — — — 2.0 Thickness of metal sheet at 45  55  80  40  45  40  45  45  45 score line (μm) Rate of change ofsheet 450 950 700 500 450 500 450 450 1200 thickness of score lineportion (μm/mm) Angle (°) of bending of  15  40  45  60 120 — — — —thinned portion Metal sheet Steel sheet Steel Steel Steel Steel SteelSteel Steel Steel sheet sheet sheet sheet sheet sheet sheet sheet Sheetthickness (mm) 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 0.19 Type CL TFSET TFS TFS TFS TFS TFS TFS Weight (mg/m²) Zirconium film¹⁾ — — — — — — —— — Hydrated Cr oxide²⁾  10  17  12  17  17  17  17  17  17 Metal Cr  14100  12 100 100 100 100 100 100 Sn 1050 — 2650 — — — — — — Ni  20 — — —— — — — — Resin film Inside surface Resin film Poly-ester Poly-esterPoly-ester Poly-ester Poly-ester Poly-ester Poly-ester Poly-esterPoly-ester Film thickness (μm) 30 12 9 30 30 30 30 30 30 Outside surfaceResin film Nylon 6 Nylon 6 Nylon 6 Nylon 6 Nylon 6 Nylon 6 Nylon 6 Nylon6 Nylon 6 Film thickness (μm) 30 12 9 30 30 30 30 30 30 Performanceevaluation Corrosion resistance EG P P EG G EG P P LV Can openability GG P G G LV P P EG Initial opening force (kg) 2.6 2.5 3.0 2.4 2.5 Can'topen 2.8 2.8 2.2 or full can openability Dropping strength EG EG EG EGEG EG EG EG EG No. of leaking cans/50 cans 0 0 0 0 0 0 0 0 0 TFS:Chrome-chrosate treated steel sheet, ET: Electrical tin plated steelsheet, CL: Ni substrate thin Sn plated steel sheet ¹⁾Shown as valueconverted to zirconium. ²⁾Amount of hydrated Cr oxide shows amount asCr. Evaluation of corrosion resistance: EG: Extremely good, G: Good, P:practical, LV: Large variation (unstable)

Note that the can openability was judged by whether the can could befully opened without problem and by measuring the opening strength. Thedropping strength was evaluated by fabricating a can with an easy-opencan lid filled with a liquid, dropping it vertically from a height of 60cm five times, and examining the state of leakage of the content fromthe easy-open can lid due to the dropping impact by n=50 times.

The evaluations of “extremely”, “good”, “practical”, and “largevariation” were made as in Table II-2.

With tab: FIG. 12(A): stay-on tab type easy-open can lid, FIG. 12(B):pull-tab type easy-open can lid, and FIG. 12(D): full-open typeeasy-open can lid.

Without tab: FIG. 12(C): tabless type easy-open can lid

TABLE II-2 Extremely Large good Good Practical variation Can Under 2.22.3 to 2.8 to 3.2 No full openability kg, full 2.7 kg, kg, full openingat (with tab) opening full opening over 3.3 Initial opening kg openingforce Possibility of full opening (without tab) Under 3.7 3.8 to 4.3 to4.7 Over 4.8 Initial kg 4.2 kg kg kg opening force Dropping None 1 to 45 to 9 10 or more strength (number of leaking cans)

Example III and Comparative Example III

The material of the easy-open can lid used was a laminated steel sheetcomprised of surface-treated steel sheet laminated with a resin film.The surface-treated steel sheet was pressed as explained above toprepare an easy-open can lid. The shape of the die used when the targetresidual thickness is 40 μm, the shape of the easy-open can lidprepared, and the results of evaluation are shown in Table III-1.

TABLE III-1 Variation in residual thickness Punch Die shoulder Punchhead Die recession Upper and lower after shoulder R R recess depth depthdie overlap processing Can Film Dropping Evalu- (mm) (mm) (mm) (mm) (mm)(μm) openability soundness strength ation 0.5 0.5 0.5 3.0 −0.5 15 — LVLV Comp. Ex. 0.5 0.5 0 10*   0.5 55 G G LV Comp. Ex. 0.5 0.5 0.5 10*  0.5 20 G G G Ex. 0.5 0.5 0 0.5 0.5 20 G G G Ex. 0.5 0.5 0.5 1.0 0.5 10 GG G Ex. 0.5 0.5 0.5 3.0 0.5 15 G G G Ex. 0.5 0.5 0.5 5.0 2.0 25 LV G GComp. Ex. 3.0 0.5 0.5 5.0 0.5 20 G G G Ex. 3.0 3.0 0.5 5.0 0.5 15 LV G GComp. Ex. The “*” in the “Die recess depth” column shows that thematerial does not contact the bottom of the recess at the center of thedie at the inside of the score line. The negative value at the “Upperand lower die overlap” column shows that the punch does not overlap thedie on the line of operation. The “—” in the “Can openability” columnshows that there is already breakage at part of the score line beforethe opening operation.

The variation in the residual thickness was found by measuring thethicknesses of the thinnest portion from examination of thecross-section of the thickness for eight points, that is, A, B, C, D, E,F, G, and H along the score line 22 shown in FIG. 14 and finding thedifference between the maximum value and the minimum value of thethickness. For the evaluation of the characteristics of the can lid 23,the soundness of the coating, the can openability, and the droppingstrength were investigated.

Note that the soundness of the coating was evaluated by a conductiontest. The can openability was evaluated by whether full opening waspossible without problem and by measuring the opening force. Thedropping strength was evaluated by preparing a can with an easy-open canlid filled with a liquid, dropping it vertically from a height of 60 cmfive times, and investigating the state of the leakage of the contentsfrom the easy-open can lid due to the dropping impact.

Further, the results of study of the variations in the residualthickness under various conditions were analyzed by the punch headrecess depth and/or die center recess depth/(punch shoulder R+dieshoulder R) in FIG. 15 and FIG. 16. From these results as well, theconcepts behind the numerical limitations of the recess depth wereverified.

Easy-open can lids were prepared by pressing a laminated steel sheetcomprised of a surface-treated steel sheet on which a resin film islaminated in the same way as in the Examples and Comparative Examplesshown in Table III-1. The shape of the die used when the target residualthickness is 40 μm, the shape of the easy-open can lid prepared, and theresults of evaluation are shown in Table III-2.

TABLE III-2 Variation in residual Upper and Punch head thickness Punchshoulder Die shoulder lower die recess Die recess after Ra Rb Ra Rboverlap depth depth processing Can Film Dropping (mm) (mm) (mm) (mm)(mm) (mm) (mm) (μm) openability soundness strength Remarks 1.0 0.5 1.00.5 0.5 0.5 0.5 10 G G G Ex. 1.0 0.5 1.0 0.5 0.5 0.5 5.0 15 G G G Ex.3.0 1.5 0.2 0.1 0.2 0.5 0.5 20 G G G Ex. 1.0 0.5 1.0 0.5 0.6 0.3 0.5 20G G G Ex. 1.0 0.5 1.0 0.5 0.5 0 0.5 15 G G G Ex. 1.0 0.5 1.0 0.5 0.5 0.510*   15 G G G Ex. 1.0 0.5 1.0 0.5 −0.5 0.5 0.5 15 — G G Comp. Ex. 1.00.5 1.0 0.5 0.5 0 10*   50 G G LV Comp. Ex. 6.0 3.0 6.0 3.0 0.5 0.5 0.525 LV LV G Comp. Ex. 5.0 4.5 5.0 4.5 0.5 0.5 0.5 25 LV LV G Comp. Ex.The “*” in the “Die recess depth” column shows that the material doesnot contact the bottom of the recess at the center of the die at theinside of the score line. The negative value at the “Upper and lower dieoverlap” column shows that the punch does not overlap the die on theline of operation. The “—” in the “Can openability” column shows thatthere is already breakage at part of the score line before the openingoperation. Ra: long diameter Rb: short diameter

From the above results, it was learned that the can lids produced in thescope of the present invention had a small variation in residualthickness and were excellent in characteristics as can lids.

INDUSTRIAL APPLICABILITY

As explained above, the easy-open can lid of the present invention usesa material obtained by laminating a resin film on a metal sheet,provides a score line by pressing without using a sharp blade, and makesthe shape near it a specific shape, whereby achievement of the goal ofthe related art, that is, realization of both can openability andcorrosion resistance, becomes possible. Further, if a steel easy-opencan lid can be commercialized, conversion to “monometal cans” wouldbecome possible, whereby products suited for recycling to deal with therecent problems of the global economy can be supplied to the market. Ofcourse, steel sheet itself is superior in economy. By making both thecan body and the can lid by steel sheet, it is possible to expectproducts which are superior in economy and can easily be reutilized asresources.

Further, the method of forming the easy-open can lid of the presentinvention forms a score line by pressing while avoiding the problem oftool life, a major problem of the related art, and enables theproduction, with a good productivity, of can lids with an extremelysmall variation in thickness over the entire length of the score lineand satisfying the properties of the can lid.

LIST OF REFERENCES

1 . . . Can lid material

2 . . . Die

3 . . . Die

4 . . . Score line (score)

5 . . . Punch

6 . . . Die

7 . . . Die

8 . . . Punch shoulder

9 . . . Die shoulder

10 . . . Can lid body

11 . . . Tear strip

12 . . . Score line (bent portion)

13 . . . Outer bead

14 . . . Outer peripheral flat portion

15 . . . Inner peripheral flat portion

16 . . . Inner bead

17 . . . Bent portion of thin portion

18 . . . Center line

19 . . . Angle of bend

20 . . . Space

21 . . . Space

22 . . . Score line

23 . . . Can lid

a . . . Width of flat portion from bench shoulder to recess of punchhead

b . . . Overlapping width of punch and die on parallel line of operation

c . . . Depth of recess of punch head

d . . . Depth of recess of die center

What is claimed is:
 1. A metal easy-open can lid superior in canopenability having a resin coating on at least one surface thereof; saideasy-open can lid having an outer peripheral flat portion, an innerperipheral flat portion, a score line portion located between said outerperipheral flat portion and said inner peripheral flat portion, with ascore line located in said score line portion; an outer bead located ata side of said outer peripheral flat portion which is opposite to saidscore line; an inner bead located at a side of said inner peripheralflat portion which is opposite to said score line; said score lineportion having a cross-sectional thickness which gradually becomesthinner at a rate of change of cross-sectional thickness of not morethan 1000 μm/mm from a location adjacent said outer peripheral flatportion to a location at said score line and said score line portionhaving a cross-sectional thickness which gradually becomes thinner at arate of change of cross-sectional thickness of not more than 1000 μm/mmfrom a location adjacent said inner peripheral flat portion to alocation at said score line.
 2. A metal easy-open can lid according toclaim 1, wherein said can lid has a can outer surface side and a caninner surface side; at least the inner bead has a shape projectingtoward the can outer surface side; a deepest recess point projectingtoward the can inner surface side of the inner peripheral flat portionis located at a range of 0.2 to 4.0 mm from a highest projecting pointof the inner bead toward the can outer surface side; the outerperipheral flat portion adjacent the score line portion is located in arange of 0.1 to 2.0 mm toward the can outer surface side from a deepestrecess point toward the can inner surface side of the inner peripheralflat portion; the inner peripheral flat portion has a cross-section witha thickness of from 0.1 to 3.0 mm; and the outer peripheral flat portionhas a cross-section with a thickness of from 0.1 to 3.0 mm.
 3. Aneasy-open can lid according to claim 1, wherein the an lid is formedfrom a metal sheet and said metal sheet is selected from the groupconsisting of a steel sheet, a surface-treated steel sheet, an aluminumsheet, and an aluminum alloy sheet.
 4. An easy-open can lid according toclaim 1, wherein the can lid is formed from a metal sheet and the canlid has a can inner surface side; wherein the resin coating on at leastthe can inner surface side of the metal sheet is a thermoplasticsaturated polyester-based resin having a thickness of 10-100 μm.
 5. Aneasy-open can lid according to claim 1, wherein the can lid is formedfrom a metal sheet and the can lid has an outer surface side; whereinthe resin coating on the can lid outer surface side is a polyamide resinhaving a thickness of 10-100 μm.
 6. An easy-open can lid according toclaim 1, wherein the can lid is opened by a method selected from:opening using a tab separating part or all of the can lid from a canbody; opening using a tab leaving part or all of the can lid on the canbody.
 7. An easy-open can lid according to claim 1, wherein the can lidis selected from a type comprising: a tabless can lid having a tearstrip provided at least at one location on the can lid for separatingpart or all of the can lid from a can body; a tabless can lid having atear strip provided at least at one location on the can lid for leavingpart or all of the can lid on the can body.
 8. A metal easy-open can lidaccording to claim 1, wherein said score line portion having saidgradually thinner cross-sectional thickness is bent.
 9. A metaleasy-open can lid according to claim 8, wherein said can lid has a canouter surface side and a can inner surface side; at least the inner beadhas a shape projecting toward the can outer surface side; a deepestrecess point projecting toward the can inner surface side of the innerperipheral flat portion is located at a range of 0.2 to 4.0 mm from ahighest projecting point of the inner bead toward the can outer surfaceside; the outer peripheral flat portion adjacent the score line portionis located in a range of 0.1 to 2.0 mm toward the can outer surface sidefrom a deepest recess point toward the can inner surface side of theinner peripheral flat portion; the inner peripheral flat portion has across-section with a thickness of from 0.1 to 3.0 mm; and the outerperipheral flat portion ha s a cross-section with a thickness of from0.1 to 3.0 mm.
 10. A method of forming an easy-open can lid superior incan openability comprising: providing an upper punch having a shoulderand a lower die having a shoulder; providing a recess at a centerlocation of the upper punch and providing a recess at center location ofthe lower die, said recesses provided for restraining a material to beshaped; pressing the material to be shaped with the shoulder of theupper punch and the shoulder of the lower die; providing at least oneupper punch shoulder R and at least one lower die shoulder R, with R ofnot more than 2.0 mm; overlapping the upper punch and the lower die on aline parallel to operation of the upper punch.
 11. A method of formingan easy-open can lid according to claim 10 further comprising: providingthe recess at the center location of the lower die with a depth of notmore than (upper punch shoulder R+lower die shoulder R)×4.
 12. A methodof forming an easy-open can lid according to claim 10 furthercomprising: providing the recess at the center location of the upperpunch with a depth of at least 0.2 mm.
 13. A method of forming aneasy-open can lid superior in can openability comprising: providing anupper punch having a shoulder and a lower die having a shoulder;providing a recess at a center location of the upper punch and providinga recess at a center location of the lower die, said recesses providedfor restraining a material to be shaped; pressing the material to beshaped with the shoulder of the upper punch and the shoulder of thelower die; providing the shoulder of the upper punch and the shoulder ofthe lower die with a shape of part of an ellipse having a long radius of0.1 to 5.0 mm and a short radius of 0.05 to 4.0 mm; overlapping theupper punch and the lower die on a line parallel to operation of theupper punch.
 14. A method of forming an easy-open can lid according toclaim 13 further comprising: providing the recess at the center locationof the lower die with a depth of not more than (long radius of upperpunch shoulder+long radius of lower die shoulder)×4.